An example embodiment of a date indicator mechanism is shown in perspective in FIG. 1 annexed to this patent application. Designated as a whole by the general reference numeral 1, this type of mechanism is for mounting in the bottom plate of a watch (not illustrated), typically a wristwatch for displaying the day of the month. Date mechanism 1 includes a top date ring 2 and a bottom date ring 3. The top date ring 2 is superposed on the bottom date ring 3. Top date ring 2 has sixteen sectors regularly distributed over its circumference. On the top face of ring 2, the sixteen sectors include successive markings that go from “17” to “31”, and a window 21, which, in the example shown, is a through aperture arranged in top date ring 2. The bottom date ring 3 also has sixteen sectors regularly distributed over its circumference. On the top face of ring 3, the sixteen sectors have successive markings from “1” to “16”. The watch will typically have an aperture through which the markings on top and bottom date rings 2 and 3 can be seen.
FIGS. 2 and 3 show more specifically constituent details of top and bottom date rings 2 and 3. Teeth 22 project radially towards the interior of top date ring 2 from a peripheral inner edge 23 of said top ring 2. Similarly, teeth 32 project radially towards the interior of bottom date ring 3 from an inner edge 33 thereof. Teeth 22 and 32 are regularly spaced along inner peripheral edges 23 and 33 of the respective date rings 2 and 3. Each tooth 22 is associated with a marking 24 or with window 21 of top date ring 2. Likewise, each tooth 32 is associated with a marking 34 of bottom date ring 3.
Date indicator mechanism 1 also includes a control wheel 4, which is for driving top and bottom date rings 2 and 3 and which completes one revolution in 31 days, driven by a pinion 61 that is driven onto the hour wheel, and gears 62 and 63 which will not be described further here.
FIG. 4 annexed to this patent application is a detailed perspective view of control wheel 4. As is clear from the Figure, control wheel 4 has first and second superposed toothings stages 41 and 42. On the periphery of each of stages 41 and 42 there is a portion fitted with successive adjacent teeth and a portion with no teeth. In the toothed portion of each of first and second toothing stages 41 and 42, the teeth are regularly spaced at an angle of 2π/31.
As is visible in FIG. 4, a reference numeral has been added to each of the teeth of toothing stages 41 and 42, indicating which sectors of top and bottom date rings 2 and 3 is controlled by that tooth. Thus, first toothing stage 41 includes teeth that will control sectors “17” to “31” and window 21 of top date ring 2. The toothless portion of first toothing stage 41 thus extends between the tooth that controls the sector corresponding to window 21 and the tooth that controls sector “17”. The second toothing stage 42 includes teeth that control sectors “1” to “16”. The toothless portion of second toothing stage 42 thus extends between the tooth that controls sector “16” and the tooth that controls sector “1”. The toothed portion of one stage is placed plumb with the toothless sector of the other stage. Thus, the teeth of the first toothing stage 41 controlling markings “17” to “31” of top ring 2 are arranged plumb with the toothless portion of second toothing stage 42. The toothless portion of first toothing stage 41 is placed plumb with the teeth of second toothing stage 42 that control markings “2” to “16” of bottom ring 3. By way of exception, the tooth of first toothing ring 41 which controls sector “1” of bottom date ring 3 is arranged plumb with the tooth of second toothing stage 42 that controls the sector corresponding to window 21 of top date ring 2.
First and second toothing stages 41 and 42 are coupled in rotation, such that a simple rotation of one revolution of control wheel 4 drives one or other of the two top and bottom date rings 2 and 3. First and second toothing stages 41 and 42 are arranged for respectively driving top date ring 2 and bottom date ring 3 via their toothed portions. Multiplier wheel sets 11 and 13 form a kinematic link between first and second toothing stages 41 and 42 and top and bottom date rings 2 and 3 respectively. Multiplier wheel sets 11 and 13 ensure that the daily rotation of control wheel 4 means that one date ring moves forward one step from one day of the month to the next.
FIG. 5 annexed to this patent application is a side view of the date indication corrector mechanism. As illustrated in this Figure, first toothing stage 41 is kinematically connected to top date ring 2 via first multiplier gear 11, third multiplier gear 13 and the top gear of a corrector gear train 9. Multiplier gear 11 includes a pinion 112 driven by the toothed part of first toothing stage 41. Multiplier gear 11 further includes a wheel 111 secured to pinion 112. Multiplier gear 13 includes a pinion 131 driven by wheel 111. Multiplier gear 13 further includes a wheel 132, secured to pinion 131, that drives the top gear of corrector gear train 9.
During the rotation of control wheel 4, date indicator mechanism 1 operates as follows. Let us assume that window 21 and marking “1” are initially placed underneath the watch aperture. The first day of the month is thus visible to the person wearing the watch. At the change, controlled by control wheel 4, from the first day of the month to the second, then from the second to the third and so on until the “16”:                the toothless part of first toothing stage 41 is opposite first wheel set 11. Top date ring 2 is thus not being driven and window 21 remains stationary, placed underneath the aperture of the watch;        the teeth of second toothing stage 42, which control the movement of markings “2” to “16”, will mesh in succession with second multiplier gear 12 and will thus drive bottom date ring 3, via its teeth 32. Thus, the dates “2” to “16” will be visible in succession in the watch aperture through window 21.        
At the change, controlled by wheel 4, from day “16” of the month, marked on bottom date ring 3 to day “17” of the month, marked on top date ring 2 then subsequently from day “17” to “31”:                the toothless portion of second toothing stage 42 is opposite second multiplier gear 12. Bottom date ring 3 is thus not being driven and the date “16” remains stationary under the aperture;        the teeth of first toothing stage 41, which control the movement of markings “17” to “31” will mesh in succession with wheel set 11, and thus drive top date ring 2, via its teeth 22. Thus, the dates “17” to “31” will be displayed in succession in the aperture.        
At the change from the date “31” carried by top ring 2 to the date “1” carried by bottom ring 3, the tooth of first toothing stage 41, which controls the movement of window 21, and the tooth of second toothing stage 42 that controls the movement of marking “1”, are simultaneously meshed with wheels sets 11 and 12 respectively, the tooth of first toothing stage 41 being superposed on the tooth of second toothing stage 42. Top date ring 2 thus pivots to place window 21 underneath the aperture, whereas bottom date ring 3 pivots to place marking “1” underneath the aperture.
An examination of FIG. 1 reveals that the position of top and bottom date rings 2 and 3 is indexed by a jumper spring 50, maintained by a spring 51. These jumper springs 50 are used for keeping date rings 2 and 3 in position and preventing them from rotating unexpectedly outside the date indication correction periods, for example via the effect of a shock. When one wishes to obtain a date mechanism with a jump duration that is as short as possible, the multiplication ratio between the control wheel and the date ring concerned must be as high as possible. Thus, the available torque at the output of the gear train that connects the control wheel to the date ring is low, such that the force exerted by the jumper spring on said date ring must be as small as possible so that it can be overcome when the date ring is made to move forward one step. However, if the retaining force exerted by the jumper spring on the date ring is low, there is a significant risk of the date ring jumping in the event of a shock and of the date indication being incorrect.